The present invention relates to coated nylon films. More particularly, the invention pertains to polyamide homopolymer or copolymer films which are coated with an aqueous dispersion of a polyurethane and a crosslinker, with subsequent crosslinking of the polyurethane. The coated film is preferably uniaxially or biaxially stretched either before or after coating. The resulting coated film has improved adhesion to biologically derived glues which find use in packaging and laminated articles such as paperboard structures which are then shaped into boxes for consumer use.
There is a significant need in a variety of industries for producing strong, durable, light weight multilayered paperboard or film laminates. Such laminates are used for producing packages for consumer products. It is desirable that these packages have a transparent nylon film layer since such a nylon layer has excellent strength and its lamination to printed paper or cardboard improves the appearance of the underlying paper or cardboard. In this use, a sheet of thin nylon film is laminated to a paper or cardboard surface by means of a suitable adhesive, thus generating a glossy, durable surface. Paper or cardboard which has been laminated in this way may be used for book covers, packaging material, boxes, and other applications. To be useful in such applications where appearance is important the nylon film must be transparent and defect-free, allowing the printed images on the paper or cardboard to be seen without distortion. It is also important that the moisture adsorption properties of nylon be unimpaired. Since nylon is moisture absorbent, it prevents curling of the laminate under the humidity conditions where the paper or cardboard also adsorbs moisture.
Packaging boxes are typically composed of one or more laminated paperboard or polymeric film sheets or objects which are held together by biologically derived glues such as animal glues. One disadvantage of using nylon laminated to cardboard for preparing decorative boxes is that the animal glue typically used to hold boxes together at the seams has poor adhesion to nylon. This weakness prevents such boxes from being used to package such heavy items as perfume or liquor bottles. Accordingly, it is desired to prepare a nylon film for use in laminating to printed paper or cardboard, which has improved adhesion to animal glues, but retains the transparency, good appearance, and moisture adsorption properties of the nylon film.
The invention provides a polyamide film having a transparent coating composition thereon comprising a water dispersion of a water dispersible polyurethane polymer and a crosslinker for the polyurethane. The aqueous dispersion is mixed with the crosslinking agent and then applied as a thin layer onto the nylon film. Water is dried from the composition and the polyurethane is cured with the crosslinking agent, usually by heating. The coated film is preferably uniaxially or biaxially stretched before or after applying the composition. Typically, a sheet of paperboard or other polymeric film substrate is adhered to the opposite side of the nylon film by use of a suitable adhesive, such as a polyethylene extrudate, and by the application of heat and/or pressure, to join the sheets into a laminated structure. The coating on the film is then contacted with a quantity of an animal glue and folded back onto itself into the desired box shape.
Water dispersions of polyurethanes and crosslinker are well known in the art. Likewise the application of certain such dispersions to polyamides is known. U.S. Pat. No. 5,354,807 shows an adhesive composition of a water dispersed polyurethane and a crosslinker. U.S. Pat. Nos. 5,532,058 and 5,494,960 teach a urethane dispersion containing a crosslinker such as aziridine which is applied to polyamide films to form a composite useful for packaging, however, such films are not stretched and therefore have less than desired strength. U.S. Pat. No. 3,300,370 teaches a polyamide article having a solid thermoplastic urethane elastomer on one surface. U.S. Pat. No. 3,870,684 teaches a urethane dispersion containing a crosslinker as a binder for paper. U.S. Pat. No. 4,695,509 teaches polyurethane composites having a crosslinker and a filler. U.S. Pat. No. 5,470,907 shows aqueous polyurethane dispersions useful for adhesives. U.S. Pat. No. 5,475,050 teaches aqueous polyurethanes containing silica. None of these teach or suggest a polyurethane dispersion containing a crosslinker such as aziridine which is on a stretched polyamide film and used to form a composite useful for high strength, transparent films for packaging and which have good adhesion to biologically derived adhesives.
Accordingly, the present invention prepares a composite by producing a layer of a coating composition comprising a polyurethane and a crosslinker on a stretched nylon film, thereby improving the adhesion of the nylon surface to animal glues without negatively affecting the transparency of the film. It is a particular advantage of this process that the coating may be applied to the nylon film prior to orientation, heated and stretched to provide this orientation, and still retain its properties of transparency, good appearance, and good adhesion to animal glue. Other ingredients may also be added to the formulation applied to the nylon film to improve other properties of the finished film, particularly including fine silica particles, which prevent the phenomenon known as blocking, and additives to improve flow and leveling.